The field of art to which this invention pertains is synthetic resins containing a hydrophilic group, said resins being soluble or dispersible in water when salted and being cathodically electrodepositable.
The coating of electrically conductive substrates by electrodeposition is an important industrial process. In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous dispersion of film-forming polymer. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous dispersion, until a desired coating is produced on the article. At the present time, the article to be coated is usually made the anode in the electrical circuit with the counter-electrode being the cathode.
For some purposes there are disadvantages in the use of anodic deposition methods. For example, anodic deposition on ferrous metals tends to discolor the electrodeposited film, and phosphate conversion coatings, which are commonly applied to a metal surface before an organic coating composition is deposited thereon, tend to be stripped from the metal under anodic deposition conditions. In addition, it is a peculiarity of anodic electrophoretic coating methods that nascent oxygen is produced at the anode, which can react with the resinous polymers to produce bubbles or voids in the deposited coatings. Such coatings are often lacking in resistive properties.
Recently, extended efforts have been put forth to develop cathodic electrodepositable compositions to alleviate the discoloration problems and to improve resistance properties. Although nascent hydrogen develops at the cathode during the cathodic electrophoretic coating process, no metal ions pass into the coating solution or at present in the deposited film. Generally, the amount of nascent hydrogen produced at the cathode does not have the same deleterious effects on the properties of the deposited film as does the nascent oxygen produced during anodic deposition.
Cathodic coating compositions generally are derived from resinous compositions containing a basic nitrogen atom which can be salted with an acid and then be dissolved or dispersed in water. Cathodic coating compositions are described in U.S. Pat. No. 3,739,435 wherein the reaction product of an epoxy resin and a secondary amine are further reacted with a monocarboxylic fatty acid and a polymer containing at least two carboxylic acid groups. The resulting product is then reacted by heating with an amino resin or a phenolic resin. The resinous reaction product is salted with an acid and dissolved or dispersed in water to form a cathodic electrodepositon bath.
U.S. Pat. No. 3,719,626 describes curable cathodically electrodepositable coating compositions made from aqueous solutions of a carboxylic acid salt of an adduct of a polyepoxide resin and allyl or dially amine.
In U.S. Pat. No. 3,804,786, water dispersible cationic resins are mde by reacting an hydroxy containing polyepoxide resin with a polyisocyanate in an amount insufficient to cross-link and gel the resin. A portion of the epoxide groups are reacted with an unsaturated fatty acid and the remaining epoxide groups are reacted with a monosecondary amine. The resulting product is then salted with a carboxylic acid and dispersed in water to form a cathodic electrodeposition bath.
Netherlands patent application No. 7,407,366 describes cathodic deposition baths made from an aqueous dispersion of a carboxylic acid salt of the reaction product of a diepoxide resin with polyfunctional amines and monofunctional amines, the polyfunctional amines acting as coupling agents and the monofunctional amines acting as terminating agents.
In U.S. Pat No. 3,947,339, cationic electrodepositable resins having improved throwing power and dispersibility are made from amine group-solubilized, epoxy resin-derived resins which contain primary amine groups. These primary amine groups are incorporated into the electrodepositable resin by reacting the epoxy-group containing resin with polyamines in which the primary amine groups are blocked by ketimine groups.
Additional cathodic electrodeposition resins are described in U.S. Pat. Nos. 3,617,458, 3,619,398, 3,682,814, 3,891,527 and 3,947,338.
U.S. Pat. Nos. 2,772,248 and 3,336,253 describe water soluble resinous compositions made from acid salts of adducts of polyepoxides and polyamines. U.S. Pat. No. 2,909,448 is directed to epoxy resin curing agents made from acid salts of polyepoxide-polyamine adducts.